Tag : geoptik

Canon 200mm 2.8 lens for widefield imaging

I have been trying out widefield imaging with the Geoptik adaptor and my Atik 460 mono camera using a relatively cheap Canon 75-300mm zoom lens. I have found that you need to step down the focal length to a more reasonable f6 or f7 to get some nice round star shapes. But really you want to me imaging at about f4 or f5. This means purchasing a more expensive camera lens like the Canon 200mm f2.8 or Canon 100mm f2.8.

I began looking at the beige Canon zoom lenses. I was interested in the 70-200mm 2.8 lens. Expensive new, but they can go for about £500-£600 used. I had been looking at the second hand market for a while. But what put me off is that I imaged in Ha and OIII at 100mm on my zoom lens and as you have to take apart the imaging train in order to change filters I found it impossible to not move the zoom when doing so.

This meant that the Ha images may have had a different Field of View compared to the OIII images. That’s usually not a problem you have with telescopes which are fixed. So a fixed prime lens did make more sense, plus it may be sharper.

As most of the widefield images I have taken are between 100mm and 200mm. I thought that a 200mm prime lens and a 100mm prime lens would do the trick. So I then started looking at the Canon 200mm f2.8 lens. I was trying to discount it as I had purchased the smaller version of the Telefokus to help me focus and the maximum diameter on those rings are around 78mm. I was worried that the rings would not fit around the barrel of the 200mm lens, but it turns out they do fit.

How I know is that I ended up buying a used Mk 1 version of the Canon 200mm prime f2.8 lens. I can’t see much difference between the Mk1 and Mk2, although the Mk1 does have an in-built lens hood.

The Canon 200mm f2.8 lens is a bit heavier – maybe more glass elements and a larger diameter is the reason.

I am yet to give the lens a go yet as it has been cloudy every night for the last week, but I’ll post some feedback and some images once I have used it.

canon 200mm f2-8 lens

IC405 Flaming Star Nebula using Geoptik Adaptor Canon Lens

This was my 3rd image using the Geoptik adaptor and my Canon 70-300mm lens. For IC405 Flaming Star Nebula I decided to move to 200mm zoom, I had tried 200mm on IC405 on the 1st night and I was not happy with the focus, or the shape of the stars. On the 2nd night my TeleFokus from Telescope Service arrived to make focusing easier (but more about that in a later post).

This time on 200mm I set the f stop to f/7.1 I was worried about going any slower than f/7.1 as I would be gathering less light and losing sharpness.

I ended up binning 2×2 and taking 300 second exposures. This image below of IC405 Flaming Star Nebula included about 20 exposures. This was also guided this time. I am still not sure about it, but may add OIII exposures to produce a bi-colour image to see if that improves it. I have also been looking at expensive Canon lenses now, like the Canon 200mm f/2.8 prime lens and the Canon 70-200mm f/4 lens.

IC405 Flaming Nebula

Orion Nebula and Horsehead Nebula

Orion nebula and Horsehead nebula in Hydrogen Alpha. I used a Canon 75-300mm lens set to 100mm at about f6 attached to a cooled CCD camera via an Geoptik adaptor. This image was consists of 15 x 300 second exposures unbinned that were also unguided.

The Orion nebula has always been tricky as it’s core is so bright. This was still a problem in this widefield image, as Orion is still bright and the Horsehead is very dark. I still used DPP in Maxim DL when processing this, then just tweaked the levels, brightness, contrast and sharpened it in Photoshop.

Orion Nebula and Horsehead Nebula

Heart and Soul Nebula IC1805

Heart and Soul Nebula IC1805 in Hydrogen Alpha. I used a Canon 75-300mm lens set at 100mm. The Canon lens is attached via a Geoptik adaptor to an Atik 460EX mono camera.

This image consisted of 10 x 600 second exposures taken on a Skywatcher EQ8 mount. The images were unguided, but I did see some drift across the exposures so I would guide next time.

It’s really difficult to focus precisely with a standard Canon lens, especially in Ha when you need greater exposures to make out the stars on the screen.

IC1805 Heart and Soul Nebula